Poly(vinyl chloride), hereafter referred to as PVC, is extensively used in wire insulation and sheathing for electric cables because of its ready availability, mechanical toughness, low cost, resistance to chemicals and weathering, and good dielectric properties.
In use, PVC is always plasticized to overcome its natural rigidity and permit it to be processed into a flexible material. In addition, other additives such as thermal stabilizers, lubricants, pigments, fillers, impact modifiers, and flame retardants are generally employed to produce PVC compositions having desired properties.
Unmodified PVC is thermally unstable, decomposing at about 150.degree. C. to release HCl and produce sites of unsaturation in the polymer which lead to chain crosslinking and scission, resulting in degradation of the polymer's properties. As the PVC decomposes, the resin becomes discolored, brittle, and finally insoluble.
To improve thermal stabilitY, various thermal stabilizers are nearly always employed in PVC compositions. The stabilizers generally used are metallic salts of inorganic and organic acids and phenols, organometallics, epoxy compounds, and phosphates. In wire and cable applications, lead stabilization systems are widely used.
Despite its thermal instability, unmodified PVC has relatively good flame retardant properties due to its high chlorine content. However, the plasticizer necessary for flexibility and good processing properties generally increases the flammability of PVC compositions, especially if used at high levels. Moreover, when PVC burns, it produces considerable smoke, and the addition of flame retardant to plasticized compositions to reduce their flammability generally increases smoke generation upon burning. Smoke is particularly dangerous since it not only contains toxic by-products of combustion and thermal decomposition of the plastic, but also restricts visibility and disorients potential victims, resulting in panic.
A further problem with burning olefin-based polymers generally and PVC in particular is that when they burn they frequently flow and drip combustible materials, thus feeding and spreading the fire.
The plastics industry has long recognized that use of PVC in interior furnishings, building materials, and coverings for wire and cable presents the hazards of flame, toxic decomposition products, and smoke in the event of fire, as discussed above. It has therefore expended very considerable efforts to find additives for PVC which reduce thermal decomposition, flammability, and smoke in the event that such PVC compositions are subjected to high temperatures or flame.
Such research has yielded knowledge of several classes of stabilizers, flame retardants, smoke suppressants, plasticizers, etc., which function in PVC, and has disclosed many useful individual chemical compounds within those classes, but it has not yet provided a full understanding of how various additives interact with other additives in PVC and with PVC itself in PVC compositions exposed to high temperatures and/or flame. Accordingly, the preparation of PVC compositions having particular combinations of properties is largely an emperical art rather than a well developed science.
In the area of PVC-based compositions for wire and cable covering applications, it is very desirable to have materials which are highly flame-retardant, tend to char rather than drip when burned or heated, produce a minimum amount of smoke when burned, and produce light-colored smoke rather than dark smoke, while still possessing the good processing properties, mechanical toughness, and resistance to environmental stresses for which PVC compositions are known.